Currently, patterning using nano-imprint technology for semiconductor device processing is being considered in order to form fine patterns on a semiconductor substrate in the process of fabricating a semiconductor device. Since a mask used in nano-imprint technology has an uneven pattern of fine (very small) features impressed as depressions therein, contamination may occur at the impression or depression portions of the mask when imprinting a mask layer of UV curable resin. For instance, a contaminant may become lodged into the recessed portion along with the UV curable resin by the pressing force of the mask against the resist layer, and block off the recessed portion of the mask where it is lodged at the time of imprinting of the resist layer. A contaminant lodged in the recesses of the mask is hard to remove by a cleaning technology that cleans the mask surface, such as binary fluid cleaning and ultrasonic cleaning. Another method can be employed to etch the mask and then lift off the contaminant, but that method changes a pattern dimension of the finely processed mask by also etching the mask.